This invention relates to a plant used to form an air-laid web of fibers and a powder on a running endless forming wire which is air-permeable and which operates mainly horizontally. The invention also comprises a suction unit positioned under the forming wire and a forming head positioned above the forming wire with a perforated base; at least one fiber feed duct to feed fiber by means of air flow from the fiber source into the forming head; and at least one powder feed duct to feed powder by means of air flow from the powder source into the forming head.
A powder, such as a superabsorbent powder (SAP), is very widely used in products which are designed to absorb liquid. Such products include, for example, sanitary napkins, diapers, and incontinence products, which are all most effectively produced using an air-laid process. During the past few years, it has become common to add SAP directly during the air-laid process. The addition of SAP during the air-laid process is particularly suitable because the forming process takes place with the aid of air only, i.e., without the use of liquid that could inadvertently activate the SAP.
SAP has a higher density than fiber (usually 0.9 g/cm3 to 1.3 g/cm3) and the same consistency as sugar (particle size 100 microns to 800 microns). Therefore, when SAP is distributed in a forming head with fiber, SAP remains in the forming head for a significantly shorter time than the fiber. Furthermore, since SAP is a powder, conventional aspiration of SAP into the forming head on a jet of air allows irregular and random distribution of the powder in the forming head.
The accumulated effect of the above-mentioned characteristics is an uneven and inhomogeneous distribution of SAP in the finished product.
The applicant""s EP Patent Application No. 94103336.7-2314 describes a forming head with rows of wings that, when running, sweep the fibers across the perforated base of the forming head. The rows of wings are positioned slightly at an angle relative to the direction of the machine to ensure that the fibers are evenly and uniformly distributed in the material to be laid on the form wire. It has been found that, even with the improved forming head, it is impossible to achieve a satisfactorily even and homogeneous distribution of SAP in the finished product. Cross-sectional distribution of fiber and SAP will normally be: plus or minus 3 percent fiber, plus or minus 7 percent SAP. If SAP is to have the optimum effect, however, it must be distributed precisely and homogeneously in the product. The product cannot optimally assimilate the SAP if the powder is not homogenously and precisely distributed, which leads to a reduction in the capacity to absorb liquid.
PCT Publication No. WO 96/07792 disclosed a method and system for producing air-laid paper webs with a specific content of powder, such as SAP. The system includes a perforated forming wire and two forming heads mounted above the forming wire. Each forming head is a bottomless box that receives a flow of air-fluidized fiber material through supply channels. The fiber material is distributed over the width of the wire inside the forming heads. A powder dispenser is used for sprinkling the powder over the entire width of the web delivered from the forming head. In one embodiment, two perforated drums are arranged in the forming head. When operating, the fiber flow is supplied to the drums that are simultaneously rotated so that the fibers are discharged through the perforated walls of the drums. Because the powder is sprinkled directly onto the forming wire, or in one embodiment, the drums, the heavier powder tends to fall down to the web faster than the lighter fibers, and therefore, it is not possible to obtain a satisfactory evenly distributed and homogeneous mixing of the powder and fibers.
PCT Publication No. WO 96/10663 discloses a plant and a process for dry-producing a web-formed product. The plant has three forming heads arranged on top of each other. Each of the forming heads has a perforated bottom that creates the ceiling of the underlying forming head, while the perforated bottom of the lower forming head is placed directly over a forming wire. In operation, the powder is introduced in an airflow led into the upper forming head and passed to the intermediate forming head through the perforated bottom of the upper forming head. Cellulose fibers are introduced into the intermediate forming head through a second airflow, The powder and cellulose fibers are then passed through the perforated bottom of intermediate forming head. Thermo-binding fibers in a third airflow are directed to the lower forming head, so that the powder, cellulose fibers, and thermo-binding fibers are passed through the perforated bottom of the lower forming head onto the forming wire. Using this process, the powder will not be mixed with fibers supplied to a forming head in one airflow and will not, therefore, result in a web with a satisfactory evenly distributed and homogeneous structure. In addition, the powder is blown into the upper forming head in a conventional way that initially produces an uneven distribution of the powder. It is not possible to produce a web having a high concentration of powder using this method as the powder will pass through the three perforated bottoms before contacting the the forming wire.
The present invention is directed to a plant for forming an air-laid web of fibers and powder including: an endless forming wire, wherein the wire is air-permeable and operates mainly horizontally; a suction unit positioned under the forming wire; a forming head positioned above the forming wire having a top and a perforated base, wherein at least one bushing is provided in the top; at least one fiber feed duct, wherein a first air flow carries a fiber source into a fiber inlet duct in the forming head; at least one powder feed duct, wherein a second air flow carries a powder source into the forming head; and at least one powder distributor located in the at least one power feed duct, configured and dimensioned to divide the second air flow into a plurality of finer jets of air, wherein the plurality of finer jets of air are aspirated into the forming head.
In one embodiment, the powder distributor is configured and dimensioned with a housing having a plurality of powder exits and at least one powder inlet duct for receiving the plurality of finer jets of air, wherein the at least one powder inlet duct is received into the at least one bushing of the forming head and connected to the powder feed duct. Preferably, the housing is configured and dimensioned in the form of a pipe having top and bottom ends and an inner cavity, wherein the bottom end has a hood, a skirt that fits tightly around the pipe, a funnel formed therein, and a base that closes off the bottom end, wherein the plurality of powder exits extend out therefrom. The housing can be mounted outside the forming head, and wherein each powder exit is connected to the powder distributor via a corresponding powder inlet duct. In one embodiment, at least one of the plurality of powder exits includes an adjustable valve, and in another embodiment, each powder exit includes an adjustable valve. The adjustable valve can be activated electromagnetically, hydraulically, or pneumatically.
The plurality of powder inlet ducts include top and bottom ends, and, in one embodiment, a portion of the plurality of powder inlet ducts have bottom ends extending past the fiber inlet duct. In another embodiment, the plurality of powder exits are positioned within the forming head.
The housing can be mounted vertically with respect to the plant, the base of the housing is facing downwards, and at least one of the plurality of powder exits is mounted inside the forming head. In this embodiment, the plurality of powder inlet ducts are configured and dimensioned to send the plurality of finer jets of air vertically into the forming head. In another embodiment, the plurality of powder inlet ducts are configured and dimensioned to send the plurality of finer jets of air horizontally into the forming head.
The first air flow has a direction that can be changed during operation. In one embodiment, the plurality of finer jets of air can be divided by an angled baffle plate. In another embodiment, the powder distributor includes a powder dosing apparatus.
Preferably, the powder source includes a superabsorbent powder.
The present invention is also directed to a device for producing a web of fibers and powder, including: a fiber feed duct for transporting a fiber feed; a powder feed duct; a powder distributor located in the powder feed duct to divide an air flow into a plurality of air flows; a forming head, having a perforated base, associated with the fiber feed duct and powder feed duct, capable of receiving and homogeneously mixing the plurality of air flows and fiber feed; and a continuous and air-permeable wire, upon which the homogenous mixture falls through the perforated base. In a preferred embodiment, the forming head further includes at least two rotating drums having perforated walls to receive and distribute the fiber feed, and a powder dosing apparatus is positioned near, or in another embodiment, inside, the at least two rotating drums.
Another aspect of the invention is directed to a method for forming a web of fibers and powder including: providing at least one fiber feed in a first air flow and at least one powder feed in a second air flow; dividing the second air flow into a plurality of finer jets of air, wherein the jets of air are guided into a forming zone; mixing the jets of air and the at least one fiber feed in the forming zone to form a homogeneous mixture, and directing the mixture downward onto a continuous and air-permeable wire.